Apparatus formed of high silicon chromium/nickel in steel in the manufacture of sulpheric acid

ABSTRACT

Apparatus for the manufacture of sulphuric acid comprising at least one gas-concentrated sulphuric acid contacting unit and a sulphuric acid heat exchanger. The contacting unit and/or the heat exchanger is formed of high silicon content austenitic steel.

This is a continuation of Application Ser. No. 07/332,433, filed on Mar.30, 1989, now abandoned, which in turn is a continuation of Ser. No.07/85,234, filed 8/13/87, now abandoned, which in turn, is a CONT. ofSer. No. 06/778,199, filed 9/20/85, now abandoned, which in turn, is aCONT of Ser. No. 06/502,500, field 6/9/83, now abandoned.

This invention relates to apparatus and processes for the manufacture ofsulphuric acid and more particularly to apparatus formed of austeniticstainless steel.

Sulphuric acid is normally manufactured by burning elemental sulphur inpreviously dried air to produce sulphur dioxide from which sulphurtrioxide is formed and absorbed in concentrated sulphuric acid where itreacts with water to form additional sulphuric acid. A similar route tosulphuric acid from metallurgical gases exists in which sulphurdioxide-containing gas is dried and the sulphur dioxide converted tosulphur trioxide. The process subsequently is as previously described.Both of these processes involve the oxidation of sulphur dioxide tosulphur trioxide followed by absorption in sulphuric acid. This isgenerally known as the contact process for the manufacture of sulphuricacid.

Some important operations involving sulphuric acid in its manufactureare drying, absorbing and cooling. Acid strengths are of the order of93%-99.5% sulphuric acid in these operations. The temperature of thesestrong or concentrated acids is generally in the range 40° C.-115° C.

Concentrated sulphuric acid is corrosive to most metals, particularly inthe higher temperature range (>100° C.), and it is highly desirable thatall of the components of a sulphuric acid manufacturing system such ascontacting towers, heat exchangers, piping, valves, pumps, distributors,and the like, that contact sulphuric acid should be of corrosionresistant materials. At the present time such systems are built of castiron, brick, various plastic, and non-metallic materials, and varioushighly expensive corrosion resistant alloys. These materials are notcompletely satisfactory, however. For example, acid brick can only befabricated in specific shapes, can swell on extended exposure toconcentrated sulphuric acid and require expensive mortars and labour forinstallation. Also, attack of the carbon steel shell beneath the brickscan produce sulphation which disrupts the shell and/or the bricks. Themetallic components used have significant rates of corrosion and unlessspecially protected or very highly alloyed, have limited life inservice.

The highly alloyed materials and the cast irons are also limited interms of fabricability, which puts limitations on plant design andresults in more flanges, fittings, cost, and areas of potential leakage.

In regards to energy recovery, temperature levels at which rates ofcorrosion are acceptable are relatively low which makes recovery of thelarge quantities of waste heat in sulphuric acid cooling systems verydifficult. Classically, energy transport in sulphuric acid plant is inthe form of steam which even at the lowest pressures used is at atemperature of over 115° C., the hottest temperature now tolerable inconcentrated acid.

One technique which has been developed over the last decade to reducecorrosion is the use of anodic protection with materials which can beelectrochemically protected. Sulphuric acid heat exchangers, whichrepresent a major cost in acid manufacturing systems are now normallyfabricated of austentic stainless steel of the 300 series (18% Cr - 8%Ni). These steels depend on an oxide film for corrosion resistance andare only useful without anodic protection at much reduced temperatures,such as for instance 55°-65° C. in 98% acid. Above this temperature inthe presence of turbulence, severe corrosion is found which cancompletely destroy equipment in a few months. Electrochemical generationof the oxide film by the use of anodic protection has been found tosubstantially reduce this corrosion and the existing stainless steelscan now be used at acid temperatures up to 120° C.-125° C.

Stainless steels are a series of alloys based on iron with generally aminimum of 12% Cr to impart corrosion resistance. The addition of nickelto the iron/chromium alloy alters the crystal structure from bodycentred cubic to face centred cubic and the resultant phase is termedaustenite. This family of materials based on Fe, Cr and Ni is termed theaustenitic stainless steels.

Significant efforts have been made to extend the range of applicabilityof austenitic alloys in high temperature, high strength acid (90%)systems. This has involved a study of a variety of normally usefulalloying elements such as molybdenum, nickel, copper, and chromium.Increased silicon content is also possible but gives only marginal, ifany, improvement in corrosion resistance :n concentrated sulphuric acid,and is not generally industrially useful as the silicon alloyed materialis significantly more difficult to make and also much more expensive.

It is known that stainless steels containing 4% silicon have acceptablecorrosion resistance to 98% sulphuric acid at moderate temperatures (Ca.80° C.) although the behaviour is not much different from the behaviourof the standard austeniric stainless steels without silicon. With theadditional costs associated with the addition of silicon and thecompensating addition of more nickel to maintain the austeniticstructure, no use has therefore been made of such alloys in thisenvironment.

Silicon containing alloys also have been tested in dilute sulphuric acidenvironments and offer unacceptable levels of corrosion and so have notbeen used in this area either.

In their prime area of utility, the austenitic stainless steelscontaining up to 5.3% silicon have been used successfully in themanufacture of concentrated nitric acid especially in the range 98%-100%acid where the corrosion resistance of the classic non-siliconcontaining grades falls off and here increasing silicon content helps inreducing corrosion, although no drastic effects of silicon level arenoted.

Addition of silicon to austenitic stainless steels has a significanteffect on the structure of the alloy produced and requires changes inthe content of other alloying elements such as nickel which must beincreased to maintain the austenitic nature of the alloy. In addition,workability and ability to fabricate the material are also compromisedby increasing the silicon content making the high silicon containingalloy expensive by comparison with lower alloys. Up to the classic levelof 4% silicon, the improvement in performance has been sufficient insome nitric acid environments to justify the additional expense but thesame conclusion has not been found for concentrated sulphuric acid.

Surprisingly, we have now found that austenitic stainless steels with arelatively high silicon content have a much greater corrosion resistanceto hot concentrated sulphuric acid than either the normal austeniticstainless steels, such as 304 and 316 of the 300 series, or the normalsilicon containing grades having up to 4% Si content. In addition,unlike any of the higher alloyed austenitic stainless steels previouslydescribed with higher chromium, nickel, or molybdenum contents, thesehigh silicon content stainless steels also are adaptable for anodicprotection.

We have further found that concentrated sulphuric acid can bemanufactured at much higher temperatures than heretofore possible withacceptable corrosion.

Accordingly, the present invention provides apparatus for themanufacture of sulphuric acid by the contact process of the typecomprising at least one gas-concentrated sulphuric acid contacting unitand a sulphuric acid heat exchanger characterised in that saidcontacting unit and/or heat exchanger is formed of an austenitic steelcontaining 4.6% to 5.8% silicon.

By the term "gas-concentrated sulphuric acid contacting unit" is meant asulphuric acid drying tower wherein water present in the air and in thesulphur dioxide used in the contact process is removed, and/or asulphuric acid absorption tower wherein sulphur trioxide is absorbed inconcentrated sulphuric acid.

The drying towers and the absorption towers are generally provided withacid distributors and mist eliminators. Some contact process plants,namely, the so-called "wet-process" plants, do not utilize a dryingtower.

A typical contact process plant in addition to having one or more dryingtowers, absorption towers and a heat exchanger also requires an acidcirculation system comprising pump tanks or reservoirs, acid pumps, anda piping and valve system. In such prior art systems the pump tank isformed typically of carbon steel with acid resistant brick lining toreduce corrosion by the hot acid. The pump is generally formed ofexpensive corrosion resistant alloy, the acid distributor and the pipingand valve system of cast iron, and the mist eliminator of a stainlesssteel framework with glass fibre elements. However, we have now foundthat such a system when formed of relatively high silicon contentaustenitic steel has enhanced corrosion resistance.

Further, as demisters are generally in the form of pads, candles orpanels, the use of a high silicon content austenitic steel frameworkwill allow lighter demisters to be used and higher temperatures to betolerated through the towers.

Prior art distributors generally comprise heavy wall cast iron pipesections with flanged, bolted connections and screw end caps with thepipes having holes fitted with polytetrafluoroethylene nozzles to reduceacid corrosion through the orifices. A distributor formed of highsilicon content austenitic steel provides an all-welded alternative withor without PTFE nozzle inserts. This allows a simple and lighterconstruction and a higher operating temperature.

Accordingly, in a further aspect the invention provides apparatus forthe manufacture of sulphuric acid by the contact process of the typecomprising

(a) at least one gas-concentrated sulphuric acid contacting unit;

(b) a mist eliminator in said gas-concentrated sulphuric acid contactingunit;

(c) an acid distributor in said gas-concentrated contacting unit;

(d) a sulphuric acid heat exchanger; and an acid circulation system bywhich acid is circulated to said contacting unit and heat exchanger;comprising

(e) a pump tank;

(f) an acid pump; and an

(g) acid piping and valve system;

characterized in that one or more of said components (a) to (g)inclusive is formed of an austenitic steel containing 4.6% to 5.8%silicon.

Preferably all of said components (a) to (g) inclusive are formed ofsaid austenitic steel. In a more preferred aspect the apparatus furthercomprises one or more of said components selected from the heatexchanger (d), pump tank (e), pump (f) and acid piping and valve system(g) provided with anodic protection means. More preferably all of saidcomponents (d) to (g) inclusive are provided with anodic protectionmeans.

In yet a further aspect the invention provides individual components ofa sulphuric acid manufacturing plant selected from the group ofcomponents consisting of a drying tower, absorption tower, misteliminator of said drying tower or absorption tower, acid distributor ofsaid drying tower or absorption tower, heat exchanger, sulphuric acidpump tank, sulphuric acid pump, and sulphuric acid piping and valvesystem, characterised in that said component is formed of an austeniticsteel containing 4.6% to 5.8% silicon.

As hereinbefore mentioned, by the term austenitic steel in thisspecification and claims is meant a steel comprising Fe, Ni and Cr insuch ratios that the steel is in the austenitic state. More specificallythe austenitic steel of use in the practice of the invention comprisesFe, Cr, Ni and Si. It is also understood that the austenitic steel ofuse in the invention may further comprise other elements, such as forexample, Mn to enhance austenitic stabilization, and other elements asalloying agents, without detracting from the utility of the invention.

It is not necessary that the whole of each individual component beformed of the austenitic steel of use in the practice of the invention.It will be appreciated, however, that it is highly desirable that all ofthose parts of the component which contact sulphuric acid, particularlyhot concentrated acid, liquid or vapour, be formed of such austeniticsteel. Thus the invention provides said components in whole or in partformed of such austenitic steel.

Preferably, the austenitic steel of use in the practice of the inventioncontains 5.0% to 5.6% Si. More preferably the steel has the composition17.5% Ni, 17.5% Cr, 5.3% Si,<0.015% C., the balance being substantiallyFe.

As briefly mentioned, hereinbefore, it has now been found thatconcentrated sulphuric acid can be manufactured at much highertemperature levels than heretofore operated at acceptable corrosionrates. Thus, absorption may be effected at a temperature in the range120°-180° C. as compared to conventional operations conducted in therange 60°-120° C. without undue corrosion.

Accordingly, in yet a further aspect the invention provides a processfor the manufacture of sulphuric acid by the contact process of the typecomprising the steps of passing air, sulphur dioxide, sulphur trioxide,or mixtures thereof through one or more gas-concentrated sulphuric acidcontacting units A, said unit being optionally provided with a misteliminator B and an acid distributor C, passing concentrated sulphuricacid through a heat exchanger D; and circulating sulphuric acid to saidcontacting unit and heat exchanger through a circulation systemcomprising a pump tank E, acid pump F, and a pipe and valve system G,characterised in that one or more of said A to G inclusive is formed inwhole or in part of an austenitic steel containing 4.6% to 5.8% silicon.

In a further aspect the invention provides a process as hereinbeforedefined wherein one or more, preferably all, of said components selectedfrom heat exchanger (D), pump tank (E), acid pump (F) and piping andvalve system (G) is anodically protected.

In a preferred aspect the temperature of the circulating sulphuric acidentering the heat exchanger is in the range 120° C.-180° C. morepreferably in the range 150° C.-170° C.

Thus, it can be readily seen that a plant and process according to theinvention may be either operated at conventional temperatures, whichresults in greatly reduced corrosion, or, operated at elevatedtemperatures to effect enhanced energy recovery with acceptablecorrosion. The advantage of either of these options is enhanced ifanodic protection is provided.

The invention also provides apparatus and processes for concentratingsulphuric acid.

Sulphuric acid is normally concentrated by boiling water from it. Onemethod is to apply direct oil-fired heat to a concentrator vessel in theform of a cast iron pot containing the acid and provided with a siliconcast iron agitator. Acid is fed to the pot from a packed column and whenconcentrated overflows through acid coolers to storage or pump tanks.The charge is typically waste or contaminated 70% acid, which isconcentrated to about 96%. Normal operational temperature is Ca. 290° C.for the acid which can give pot wall temperatures of the order of 350°C. and above depending on the mode of heating. The main drawback of sucha concentrator system is the corrosion and cracking of the pot. Anodicprotection is optionally applied to prolong pot life.

Another system used for concentrating acid is to use a concentratorvessel in the form of a drum formed of lead-lined mild steel having aninner brick lining. Concentration is achieved by countercurrent contactwith hot gas at a temperature of 600°-675° C. played on and into theacid. The acid charge is generally 70% acid and it leaves the drum as93% acid at a temperature of 230° C. The hot acid product is generallyused to pre-heat the incoming acid by means of a tantalum heatexchanger. The drum concentrator vessel is generally satisfactory butexpensive and subject to brickwork erosion/corrosion. It is generallyinefficient and can cause problems with contaminated gaseous effluent.

Yet another sulphuric acid concentration process is used wherein theboiling is usually carried out by inserting a heating element, typicallycontaining tantalum boiler tubes, inside the concentrator vessel used toboil the liquid. It is found that sulphuric acid at a temperatureexceeding 190° C. will usually attack tantalum. Therefore sulphuric acidmust be concentrated at a temperature such that the skin temperature ofthe tantalum tubes is less than 190° C., and this temperature limitationnecessitates a substantial reduction of pressure in the evaporator.Because of the reduced pressure in the evaporator, a complex vacuumsystem is used to condense the water boiled off the acid. The vacuumsystem usually includes a steam ejector to increase the pressuresufficiently to condense the water vapor. The capital and steamoperating costs of the vacuum system and of the tantalum heater tubesare high, particularly in plants used to produce higher concentrationacid such as 96% sulphuric acid. Accordingly, it is an object of theinvention to provide apparatus and a process for concentrating sulphuricacid in a concentrator so that reduced maintenance due to corrosionproblems is achieved and that higher operating temperatures than heretobefore used can be attained to allow higher strength acid to beproduced. In its broadest aspect this invention provides apparatus forconcentrating sulphuric acid from a strength of 85%, preferably 90% acidof the type comprising a concentrator vessel characterised in that theconcentrator vessel is formed in whole or in part of an austenitic steelcontaining 4.6% to 5.8% silicon. By "concentrator vessel" is meant apot, drum, vacuum evaporator vessel, or the like wherein sulphuric acidin the vessel is heated either directly or indirectly, by a directflame, heating coil or jacketed steam pipe or electrical, heatexchanger, or via a thermosyphon loop.

It is to be understood that the apparatus according to the invention isalso of use in acid concentrator systems wherein feed acid is of <85%strength, typically 70%, which is pre-concentrated to 85%, preferably90%, strength prior to contacting the austenitic steel of theconcentrator vessel.

The sulphuric acid charge is generally pre-heated in acid concentratorsystems. Preferably the austenitic steel contains 5.0% Si to 5.6% Si.More preferably, the austenitic steel has the composition 17.5% Ni,17.5% Cr, 5.3% Si, <0.015% C., the balance being substantially Fe.

The apparatus may optionally be provided with anodic protection means,and air extraction means.

The concentrator apparatus according to the invention also findsapplicability in the destructive heating of organic matter in sulphuricacid waste liquors contaminated with the organic matter, where corrosionof the concentrator vessel is a problem as hereinbefore described.

Further objects and advantages of the invention will appear from thepreferred embodiments now described by way of example only withreference to the accompanying drawings in which:

FIG. 1 shows diagrammatically a plant for the production of sulphuricacid by the contact process as known in the prior art and, whenmodified, according to the invention;

FIG. 2 shows diagrammatically a heat exchanger of use in the manufactureof sulphuric acid as known in the prior art and, when modified,according to the invention;

FIG. 3 is a schematic view showing a heat exchanger with an anodicprotection system installed therein as known in the prior art and, whenmodified, according to the invention;

FIG. 4 is a sectional view taken along the lines 4--4 of FIG. 3;

FIG. 5 is a vertical sectional view of a conventional drying tower ofuse in the sulphuric acid plant of FIG. 1;

FIG. 6 is the drying tower of FIG. 5 modified according to theinvention;

FIG. 7 is a vertical sectional view of a conventional absorption towerof use in the sulphuric acid plant of FIG. 1;

FIG. 8 is the absorption tower of FIG. 7 modified according to theinvention; and

FIG. 9 is a diagrammatic view of a portion of a typical prior artsulphuric acid concentration system as known in the prior art and, whenmodified, according to the invention.

The apparatus shown in FIG. 1 includes as gas-concentrated sulphuricacid contacting units three towers, namely a drying tower 10, anintermediate absorption tower 11, and a final absorption tower 12. Eachof these towers has a gas inlet A at the bottom and a gas outlet B atthe top. Each tower has an acid inlet P at the top, and an acid outlet Qat the bottom. Each of the towers is formed of carbon steel and linedwith acid-resistant brick lining C. In the upper part of each tower is acast iron acid distributor D above which is a mist eliminator E formedof glass fibre contained in a stainless steel frame. Each tower isfilled with a ceramic packing F supported by a ceramic support S andthrough which gas or air and acid percolate to produce a full andintimate contact therebetween. Also shown in the drawing are threeanodically protected stainless steel sulphuric acid heat exchangers G inwhich heat is rejected to cooling water.

The drawing also includes an acid circulation system comprising pumptanks or reservoirs H, circulating pumps J, and a piping and valvesystem K.

The acid outlets 10Q, 11Q, and 12Q drain into pump tanks 10H, 11H, 12H,respectively, which are formed of carbon steel with acid resistant bricklinings. Pumps 10J, 11J, 12J are formed of expensive corrosion resistantalloy and cast iron and circulate the acid from the tanks 10H, 11H, 12Hthrough heat exchangers 10G, 11G, 12G, to the towers 10, 11, 12, throughthe cast iron piping and valve system 10K, 11K, 12K. The piping andvalve system also includes water addition streams L, by which the waterrequirements for the acid produced are met, and acid transfer lines M.

In operation the process gas circulation system of the apparatus followsnormal practice. Air enters the drying tower 10 through inlet 10A and isdried by contact with a countercurrent stream of hot concentratedsulphuric acid which enters the tower through inlet 10P and distributedacross the packing 10F by distributor 10D. Dried air leaves the towervia outlet 10B and sulphuric acid by outlet 10Q. Mist entrained in thedried air is removed in mist eliminator 10E. Sulphur is burned with thedry air in a sulphur burner (not shown) to produce sulphur dioxide. Theprocess gas, which is now a mixture of air and sulphur dioxide, thenpasses through a catalytic converter (not shown) where the majority ofthe sulphur dioxide is converted into sulphur trioxide. The process gas,laden with SO₃ and unconverted SO₂, enters the intermediate absorptiontower 11 through inlet 11A. The SO₃ is absorbed from this intermediategas stream by countercurrent contact in packing 11F with a stream ofconcentrated sulphuric acid which enters the tower via system 11K andinlet 11P and distributed therein by distributor 11D. The absorbed SO₃exits through outlet 11Q and reacts with water injected from 11L intotank 11H to form sulphuric acid. The gas exits from outlet 11B, withessentially all of the SO₃ having passed into the acid. The effluent gasfrom the intermediate absorber then passes through a second catalyticconverter (not shown) where almost all of the SO₂ present is convertedinto SO₃. The effluent gas from the second converter then enters thefinal absorption tower 12, where the last remnants of SO₃ are absorbedby the acid circulating in tower 12 and reacted with water injected intotank 12H from 12L to form sulphuric acid. The gas finally exhausts tothe atmosphere through a stack not shown).

As mentioned hereinabove acid strengths and levels in the system areregulated in tanks H through the water addition points L and thetransfer lines M to ensure proper acid concentrations and levels forgood absorption and drying.

In the drying operation, conventional practice is to use drying acidstrengths of between 93% and 98% at entering temperatures of around 50°C. for the 93% acid and up to 80° C. for the 98% acid; the temperaturebeing set by the vapour pressure of the acid. Maximum temperatures ofthe acid leaving the drying tower are set by the corrosivity of the acidon the equipment on the one hand, and the need to have sufficient acidflow for proper gas-concentrated sulphuric acid interaction on theother. Typical maximum acid temperatures range from 70° C. for 93% acidto 90°-95° C. for 98% acid. Under these drying tower conditionsequipment life is generally of the order of five years. The acidstrength of the product taken out of pump tank 10H along product line10M is 93%-98%.

The absorption towers use 97.5%-99.5% acid where the total vapourpressure over the acid is the lowest. Acid temperatures range from50°-85° C. for acid entering the towers and up to 120° C. for acidleaving the towers. Irrigation conditions in the towers normally limitthe temperature rise in the absorption systems to 35° C. or less,especially in the intermediate absorber. Under these conditions,corrosion of cast iron is modest and acceptable equipment lifetimes areachievable.

Piping and valve systems, distributors and acid coolers heat exchangers,are all sensitive to acid turbulence and velocity. Accordingly,velocities are normally set at under 1.4 meters per second. In addition,all points of high velocity or turbulence such as valves, orifices, pumpimpellers, and the like, are normally installed in more expensivematerials such as a fluorogenerally carbon polymer, e.g. TEFLON^(*),higher alloys such as Hastalloy "C"^(*), or Lewmet^(*), or a ceramicmaterial, all of which complicate and add significant cost to theassembly.

FIG. 1 is used to also show a flow sheet of an acid plant modifiedaccording to the invention wherein the drying tower 10, absorptiontowers 11, 12, acid distributors D, mist eliminators E, heat exchangersG, pump tanks H, circulating pumps J and the piping and valve system Kare formed of a relatively high silicon austenitic stainless steelhaving the composition 17.5% Ni, 17.5% Cr, 5.3% Si, <0.015% C, thebalance being Fe. In this preferred embodiment, the use of thisrelatively high silicon austenitic steel in the piping and valve systemallows of smaller diameter piping because of higher corrosionresistance. Also, the heat exchangers or coolers G are not provided withanodic protection means since operation at the conventional temperatureshereinbefore described permits anodic protection to be dispensed with;and thus provides further simplification of the plant. In thealternative, the corrosion resistance of the austenitic steel allows anincrease in operational temperature to 130° C., which is beyond existingpiping, valve and other equipment limits, with acceptable corrosionrates.

The following Table I shows the corrosion effect of various strengths ofsulphuric acid at various temperatures upon conventional materials andon A611 the preferred material of use in the practice of the invention.The materials were not anodically protected.

Other specific austenitic steels of use in the practice of the inventionhave composition 17.97% Cr, 17.15% Ni, 5.09% Si, 0.74% Mn, 0.013% C, thebalance being substantially Fe, (from BOHLER, VIENNA, AUSTRIA) and 17.5%Cr, 17.8% Ni, 5.55% Si, 0.74% Mn, 0.013% C and the balance beingsubstantially Fe.

                  TABLE I                                                         ______________________________________                                                   Corrosion rate mpy (mils per year)                                 Temperature °C.                                                                     A611*       316L**  Cast Iron                                    ______________________________________                                                   in (A) 98.2% sulphuric acid                                         63          --          --      40                                           100          --          --      60-70                                        105          0.84         64     --                                           115          4.4         101     --                                           125          4           147     --                                           135          32          --      --                                           145          64          --      --                                                      in (B) 93.5% sulphuric acid                                         85          10          163     --                                           ______________________________________                                         *17.5% N, 17.5% Cr, 5.3% Si, 0.015% C and the balance is substantially Fe     **A conventional austenitic stainless steel of composition 18% Cr, 10% Ni     3% Mo, <0.2% Si, <0.03% C, the balance being Fe.                         

Corrosion studies with 70% sulphuric acid show that at 50° C., A611 hada high corrosion rate which was also higher than that of 316L under thesame test conditions.

The above results show, quite unexpectedly, the very high favourablecorrosion resistance to concentrated sulphuric acid of A611 over castiron and 316L. This is in contract to results obtained in lessconcentrated acid for A611 and 316L.

Table II shows the comparative corrosion effect of various strengthssulphuric acid at various temperatures on A611 in polarized corrosiontests, i.e., simulated anodic protection tests conducted at steadypotential for long terms. The corrosion rates are dependent on thepotential applied during the test. The results obtained are for valuesof potential being a reasonable working range for anodic protection,i.e., -100mV to +200mV.

                  TABLE II                                                        ______________________________________                                        Sulphuric                   Corrosion                                         Acid Strength %                                                                              Temperature °C.                                                                     Rate mpy                                          ______________________________________                                        98.5           100          0.77                                              "              274          5.7                                               98.2           115          0.26                                              93.5            65          0.05                                              "               75          0.20                                              "               85          1.15                                              "               95          0.91                                              "              200          45                                                ______________________________________                                    

Table II shows the favourable effect of anodically protecting A611 whenit is to be used in contact with concentrated sulphuric acid.

Thus, with reference to FIG. 1, a more preferred process according tothe invention incorporating advantages to enhance energy recovery willnow be described.

The plant components are formed of the high silicon austenitic stainlesssteel having the composition 7.5% Ni, 17.5% Cr, 5.3% Si, <0.015% C, thebalance being Fe, as hereinbefore described, except that now the heatexchangers 11G, 12G, pump tanks 11H, 12H, and all the piping and valvesbetween towers 11, 12 and heat exchangers 11G and 12G respectively areprovided with anodic protection means (not shown). Further, in view ofthe difficulty in providing anodic protection to the absorption towers11, 12, and of the high temperatures experienced at the lower parts ofthese towers, the inner lower parts are brick lined.

In this embodiment air drying is carried out in drying tower 10 toachieve proper drying at temperatures in the upper range of thatpreviously described. The drying operation is carried out usingconcentrated acid in the 98% range to allow of the use of highertemperatures more suitable for energy recovery, i.e., 80° C. to 110° C.

In the cooler areas of the intermediate and final absorption systems,namely, the upper part of towers 11, 12, distributors 11D, 12D, misteliminators 11E, 12E, and the piping between the heat exchangers 11G,12G and towers 11, 12 respectively, the acid temperature is of the orderof 130° C., which is near the maximum temperature allowed for use with anon-anodically protected high silicon austenitic steel.

In the hotter zones of the absorption systems, i.e., those areas wherethe components are anodically protected as hereinbefore described,namely, the heat exchangers 11G, 12G, tanks 11H, 12H, and the pipingbetween towers 11, 12, and heat exchangers 11G, 12G, the acidtemperatures are in the range 160° C.-170° C.

The direct result of operating the process according to this embodimentat the stated temperatures is the raising of the energy level in theacid system to levels compatible with energy recovery of significantvalue. The overall energy efficiency of this sulphuric acid plant can beraised from a conventional level of 60-65% to a level ca 90%.

Reference is now made to FIG. 2, which shows diagrammatically a typicalprior art heat exchanger lC0 of the kind presently in commercial use inthe manufacture of sulphuric acid. Heat transfer equipment such as shelland tube heat exchangers are used to cool sulphuric acid, wherein,generally, the corrosive acid passes around the heat exchanger tubingwhile water passes through the tubes to cool the fluid circulatingoutside the tubes, i.e., where the corrosive fluid is on the shell sideof the heat exchanger.

The heat exchanger 100 includes an outer shell 120 divided into a waterinlet box 14, a water outlet box 16, and a cooling section 18, the threesections being separated by tube sheets 20, 22. Heat exchanger tubes 24extend between the tube sheets to carry water therebetween. The shell,tube sheets and tubes are commonly made of standard grades of austeniticsteels, which in the absence of anodic protection will corrode at anunacceptably rapid rate in the presence of hot concentrated sulphuricacid. The water inlet box 14 and outlet box 16 are formed of carbonsteel. Only two tubes 24 are shown in FIG. 2, but in practice there maybe more than 1,000 of the tubes 24, packed very closely together withsmall clearances (typically 0.25 to 0.5 inches) therebetween. .Coolingwater enters the water inlet box 14 via inlet 26, flows through thetubes 24, and exits from the water outlet box 16 via outlet 28. Hot acidenters the cooling section 18 via an acid inlet 30 and leaves via anacid outlet 32. Conventional baffles 34 are provided to ensure that theacid flows through a tortuous path in the cooling section 18 for maximumcooling.

In operation, when the heat exchanger is not provided with anodicprotection means, the tube wall temperatures generally are of the orderof 30° C. for 93% acid and 55° C. for 98% acid.

Reference is again made to FIG. 2 which now represents a heat exchangeraccording to the invention wherein shell 120, tube sheets 20, 22, tubes24 and baffles 34 are formed of relatively high silicon austeniticstainless steel having the composition 17.5% Ni, 17.5% Cr, 5.3% Si,<0.015% C, the balance being substantially Fe. In operation, thisembodiment according to the invention when not provided with anodicprotection means has tube wall temperatures of the order of 80° C. for93% acid and 130° C. for 98% acid.

At temperatures where corrosion rates would be too high to use aconventional austenitic stainless steel heat exchanger, anodicprotection is applied.

A conventional anodic protection system is shown in FIGS. 3 and 4. Thereference numerals and materials described hereinbefore for the heatexchanger of FIG. 2, i.e., without anodic protection means, are appliedand incorporated herein and to FIGS. 3 and 4. FIGS. 3 and 4 also includean elongated cathode 36, typically thirty feet or more in length, whichis inserted into the heat exchanger100 from one end thereof. The cathode36 consists of a central core 38 of relatively acid resistant alloy,available commercially under the name Hastelloy C276^(*), surrounded byan insulating sheath 40 of polytetrafluoroethylene perforated withnumerous holes 42 to allow the acid in the cooling section to contactthe metallic cathode core 38. The sheath 40 prevents grounding of thecathode core 38 on the metal parts of the heat exchanger and avoidstranspassivity on baffles and tube sheets in close proximity to thecathode. The cathode 36 is supplied with current from the negativeterminal 44 of a DC power supply 46, the positive terminal 48 beingconnected directly to the shell 120. The power supply 46 is controlledby an automatic controller 50 which in turn is controlled by thepotential derived from a reference electrode 52. *denotes Trade Mark

In operation, such conventional heat exchangers provided with anodicprotection means can survive tube wall temperatures of the order of 70°C. for 93% acid and 100° C. for 98% acid.

Reference is again made to FIGS. 3 and 4 which also represent a heatexchanger provided with anodic protection means according to theinvention wherein shell 120, tube sheets 20, 22, tubes 24 and baffles 34are formed of austenitic stainless steel having the composition 17.5%Ni, 17.5% Cr, 5.3% Si, <0.015% C, the balance being substantially Fe.

In operation, this embodiment has tube wall temperatures of the order of120° C. for 93% acid and 180° C. for 98% acid. Thus, the use of thishigh silicon steel in the fabrication of anodically protected heatexchangers extends the usable temperature range of operation of suchsulphuric acid coolers and permits increased energy recovery from thehot acid.

In an alternative embodiment of the invention the cathode could also beformed of this high silicon austenitic steel for some applications.

FIG. 5 shows a conventional drying tower 10 constructed and operated aspart of the sulphuric acid plant as hereinbefore described withreference to FIG. 1.

FIG. 6 shows the drying tower 10 of FIG. 5 modified according to theinvention wherein the tower 10, distributor 10D, mechanical pad demister10E and support 10S are formed of austenitic steel having thecomposition 17.5% Ni, 17.5% Cr, 5.3% Si, <0.015% C, the balance beingsubstantially Fe; and wherein there is no acid brick lining in the lowerpart of the tower.

This embodiment is operated as part of the sulphuric acid plant ashereinbefore described with reference to FIG. 1.

The absorption tower of FIG. 7 is constructed and operated as anintermediate tower 11, (and as a final tower 12) as part of thesulphuric acid plant as hereinbefore described with reference to FIG. 1.

FIG. 8 shows the absorption tower of FIG. 7 modified according to theinvention wherein the tower 11, distributor 11D, candle type demister11E, and support 11S are formed of austenitic steel having thecomposition 17.5% Ni, 17.5% Cr, 5.3% Si, <0.015% C, the balance beingFe; and wherein there is no acid brick lining in the tower.

In an alternative embodiment, in addition to the use of the high siliconaustenitic steel the tower is lined with acid brick. This allows highertemperatures to exist in these towers than when the high silicon steelis used alone. The absorption towers, according to the invention, areoperated as part of the sulphuric acid plant as hereinbefore describedwith reference to FIG. 1.

Reference is now made to FIG. 9, which as indicated shows a portion of asulphuric acid concentration system which comprises a vacuum evaporationsystem and which is in use at the present time by the assignee of thepresent invention. The apparatus comprises a series of vacuum stages toevaporate water from the weak acid to achieve the desired productconcentration. The system includes as a concentrator vessel a firststage evaporator 110 which receives pre-heated feed acid via a conduit112 from a pre-heater, not shown. The bottom of the evaporator 110 isequipped with a thermosyphon loop 114. The loop 114 has a first leg 116for downwardly flowing acid and a second leg 118 for upwardly flowingacid, the bottoms of the legs being connected together by a conduit 120.The tops of both legs are connected to the bottom of the evaporator 110at or just below the surface of the acid 122 therein.

Acid in the leg 118 is heated by a tantalum heater 124 containingtantalum tubes (not shown individually) and into which hot high pressuresteam is injected at 126 and condensate withdrawn at 128. Convectioncurrents cause a flow of acid downwardly through leg 116 and thenupwardly through 118 past the heater 124 where heat is transferred tothe acid. Water vapour evaporated from the acid leaves the evaporator110 via a conduit 130 and is condensed in a direct contact condenser132. A steam ejector 134 is used to extract air and thus maintain thedesired vacuum on the system. A demister pad 136 is incorporated intothe evaporator 110 to remove acid drops and mist particles.

The sulphuric acid which has been partially concentrated in evaporator110 flows by gravity via conduit 138 to the next stage evaporator 140which is similar in design to the evaporator 110 but which operates at alower absolute pressure. Typically a booster ejector 141 will berequired to raise the pressure in the final stage condenser to achievesatisfactory condensation.

The evaporator 110 and piping used are typically borosilicate glass(Pyrex, trade mark) or glass lined steel to prevent attack by the acid.

The cold feed acid (>85%) is generally pre-heated by the hot productacid (93%) using a tantalum heat transfer surface in a glass orglass-lined steel shell heat exchanger. The product acid after beingcooled to storage temperature in a final water cooled heat exchanger istransferred to a pump tank or reservoir by means of an associated pipingand valve system.

Anodic protection is preferably provided to all equipment contacting theacid.

FIG. 9 is used to also show an acid concentration system modifiedaccording to the invention wherein the evaporator vessel 110 andassociated thermosyphon loop 114, heat exchangers, pump tank and theassociated piping and valve system are formed of A611 austenitic steel.

The process is operated as hereinabove described with reference to theprior art except that higher operating temperatures (>300° C.) can beattained with acceptable corrosion rates to allow higher strength acid(98%) to be produced.

In an alternative embodiment the complex and expensive vacuum systemsand tantalum heating elements and exchangers can be dispensed with. Thehigher operating temperatures reduces the need for a vacuum system, andreplacement of tantalum components with A611 allows acceptable corrosionrates.

In a further embodiment a concentrator system incorporating A611components could be used in series with one of the prior artconcentrators to concentrate acid up to higher strengths than ispresently possible, i.e., up to 98% acid at 290°-300° C.

We claim:
 1. In apparatus for the manufacture of sulphuric acid by thecontact process of the type comprising a gas-concentrated sulphuric acidcontacting unit and a sulphuric acid heat exchanger, the improvementwherein the contacting unit and/or heat exchanger is formed at least inpart of a corrosion resistant wrought austenitic steel consistingessentially of 17.5% Cr, 17.5% Ni, 4.6-5.8% Si, the balance being Fe. 2.Apparatus as claimed in claim 1 wherein said contacting unit is asulphuric acid drying tower.
 3. Apparatus as claimed in claim 1 whereinsaid contacting unit is an absorption tower.
 4. In apparatus for themanufacture of concentrated sulphuric acid by the contact process of thetype comprising:(a) a gas-concentrated sulphuric acid contacting unit;(b) a mist eliminator in said gas-concentrated contacting unit; (c) anacid distributor in said gas-concentrated contacting unit; (d) asulphuric acid heat exchanger; and an acid circulation systemoperatively connected with said contacting unit and heat exchanger bywhich acid is circulated to said contacting unit and heat exchanger,said acid circulation system comprising: (e) a pump tank; (f) an acidpump; and an (g) acid piping and valve system; the improvement whereinone or more of said components (a) to (g) inclusive is formed at leastin part of a corrosion resistant wrought austenitic steel consistingessentially of 17.5% Cr, 17.5% Ni, 4.6-5.8% Si, the balance being Fe. 5.A heat exchanger suitable for use in the manufacture of sulphuric acidby the contact process, said exchanger being formed in whole or in partof a corrosion resistant wrought austenitic steel consisting essentiallyof 17.5% Cr, 17.5% Ni, 4.6-5.8% Si, the balance being Fe.
 6. Agas-concentrated sulphuric acid contacting unit in the form of a dryingtower or an absorption tower suitable for use in the manufacture ofsulphuric acid by the contact process, said contacting unit being formedin whole or in part of a corrosion resistant wrought austenitic steelconsisting essentially of 17.5% Cr, 17.5% Ni, 4.6-5.8% Si, the balancebeing Fe.
 7. A drying or absorption tower acid distributor suitable foruse in the manufacture of sulphuric acid by the contact process, saiddistributor being formed in whole or in part of a corrosion resistantwrought austenitic steel consisting essentially of 17.5% Cr, 17.5% Ni,4.6-5.8% Si, the balance being Fe.
 8. A drying or absorption tower misteliminator suitable for use in the manufacture of sulphuric acid by thecontact process, said mist eliminator being formed in whole or in partof a corrosion resistant wrought austenitic steel consisting essentiallyof 17.5% Cr, 17.5% Ni, 4.6-5.8% Si, the balance being Fe.
 9. A storageor pump tank suitable for use in the manufacture of sulphuric acid bythe contact process, said tank being formed in whole or in part of acorrosion resistant wrought austenitic steel consisting essentially of17.5% Cr, 17.5% Ni, 4.6-5.8% Si, the balance being Fe.
 10. An acid pumpsuitable for use in the manufacture of sulphuric acid by the contactprocess, said pump being formed in whole or in part of a corrosionresistant wrought austenitic steel consisting essentially of 17.5% Cr,17.5% Ni, 4.6-5.8% Si, the balance being Fe.
 11. An acid valve suitablefor use in the manufacture of sulphuric acid by the contact process,said valve being formed in whole or in part of a corrosion resistantwrought austenitic steel consisting essentially of 17.5% Cr, 17.5% Ni,4.6-5.8% Si, the balance being Fe.
 12. In apparatus for concentratingsulphuric acid from a strength of 90% acid comprising a concentratorvessel, the improvement wherein said vessel is formed at least in partof a corrosion resistant wrought austenitic steel consisting essentiallyof 17.5% Cr, 17.5% Ni, 4.6-5.8% Si, the balance being Fe.